A 5-year-old boy is evaluated for apparent ongoing hemolysis. His hemoglobin is 9.5 g/dL, with 8% reticulocytes and MCV 87 fL. Platelets and leukocytes are normal. His direct antiglobulin test (DAT) is negative. No cold agglutinin is detectable. His family history is negative for blood disorders. Peripheral smear reveals basophilic stippling in 10% of the red blood cells. Given these findings, which of the following blood disorders is most likely?

Correct Answer: E

Rationale: In this scenario, the most likely blood disorder is option E) Lead toxicity. This is indicated by the presence of basophilic stippling in the red blood cells, which is a classic feature of lead poisoning. Lead inhibits several enzymes involved in heme synthesis and can lead to hemolysis and anemia. The negative DAT and absence of cold agglutinins rule out autoimmune hemolytic anemia. Option A) Rh-null disease is a rare condition where the individual lacks Rh antigens on their red blood cells, leading to hemolytic anemia. However, this is not consistent with the findings in the case. Option B) Hereditary pyropoikilocytosis is a rare genetic disorder characterized by red blood cell abnormalities, but it does not present with basophilic stippling. Option C) Glucose phosphate isomerase deficiency would typically show Heinz bodies on the peripheral smear, which are not mentioned in the case. Option D) Unstable hemoglobin disorders would show hemolysis with Heinz bodies, which are not described in this patient. Educationally, this case highlights the importance of recognizing specific morphological features on peripheral smears to differentiate between various hematologic disorders. Understanding the clinical significance of basophilic stippling can lead to the correct diagnosis of lead toxicity in cases of suspected hemolysis.

Correct Answer: E

Rationale: The correct answer is D: Vomiting center. Ondansetron and aprepitant are antiemetic medications that work by inhibiting signaling in the vomiting center of the brain. The vomiting center is located in the medulla oblongata in the brainstem, which plays a crucial role in triggering the reflexes that lead to nausea and vomiting. By blocking signals in the vomiting center, these medications effectively prevent and control nausea and vomiting. Incorrect choices: A: The vestibular system is responsible for balance and spatial orientation, not for controlling nausea and vomiting. B: The cerebral cortex is involved in higher brain functions such as thinking and decision-making, not in regulating nausea and vomiting. C: The hypothalamus regulates various physiological processes like temperature regulation and hormone release, but it is not directly involved in controlling nausea and vomiting.

Correct Answer: E

Rationale: The correct answer is not listed in the given options, so let's analyze the options provided. A) Free erythrocyte protoporphyrin and IgG levels: This option is not the best choice because these tests are not specific to distinguish between autoimmune hemolytic anemia and hereditary spherocytosis. B) Hemoglobin electrophoresis and direct antiglobulin test (DAT): This option is a better choice as the direct antiglobulin test (DAT) can help differentiate autoimmune hemolytic anemia from hereditary spherocytosis by detecting the presence of antibodies on the surface of red blood cells in autoimmune hemolytic anemia. C) Lactate dehydrogenase (LDH) and modified Russell viper venom test: While LDH levels can be elevated in hemolysis, the modified Russell viper venom test is not typically used to distinguish between autoimmune hemolytic anemia and hereditary spherocytosis. D) Red cell distribution width (RDW) and mean corpuscular hemoglobin concentration (MCHC): These parameters are not specific tests to differentiate between autoimmune hemolytic anemia and hereditary spherocytosis. In an educational context, it is essential to understand the specific laboratory tests used to differentiate between different types of anemia. Understanding the rationale behind selecting the appropriate tests based on clinical presentation and suspected diagnosis is crucial for accurate diagnosis and appropriate management of hematologic disorders. The direct antiglobulin test (DAT) is a key test in distinguishing autoimmune hemolytic anemia from hereditary spherocytosis due to its ability to detect the presence of antibodies on the surface of red blood cells.

Correct Answer: D

Rationale: The correct answer is D: Dexamethasone exposure. Dexamethasone is a corticosteroid commonly used in the treatment of acute lymphoblastic leukemia. It can cause avascular necrosis of the hip due to its effect on bone metabolism, leading to increased hip pain during weight-bearing activities. This is a known side effect seen more commonly in patients exposed to dexamethasone. Incorrect Choices: A: Younger age at diagnosis - Age is not a known risk factor for avascular necrosis in this context. B: Non-White race - Race is not a known risk factor for avascular necrosis in this context. C: Low body-mass index - Body mass index is not a known risk factor for avascular necrosis in this context.

Correct Answer: B

Rationale: The correct answer is B: Hyperleukocytosis; leukapheresis or manual exchange transfusion and initiation of induction chemotherapy. In this scenario, the patient's symptoms and lab findings are consistent with acute myeloid leukemia (AML) with leukostasis, causing hypoxia and interstitial infiltrates. Leukapheresis or manual exchange transfusion is crucial to rapidly reduce the high white blood cell count, which can help alleviate symptoms and prevent complications like tissue hypoxia. Initiation of induction chemotherapy is also essential for long-term management of AML. Rationale for why other choices are incorrect: A: Hyperleukocytosis alone without leukostasis does not typically require immediate leukoreduction, as in this case. Induction chemotherapy should be initiated promptly to address the underlying AML. C: COVID-19 infection would not typically present with such profound leukocytosis and monocytic morphology. Convalescent plasma and prednisone are not